325
There are several pieces of evidence that support the role of bacteria in sulfur
cycle. For instance, in a study by Raina et al. (2009), they suggested the role of
coral-associated bacteria in providing sulfur to the host by degrading sulfur compounds such as dimethylsulfoniopropionate (DMSP), dimethyl sulfide (DMS), and
acrylic acid. Moreover, these sulfur compounds may have a role in structuring bacterial communities beneficial for the health of corals.
Coral-associated bacteria also participate in scavenging limiting nutrients such
as iron by siderophore production, amino acids, and vitamins for the host. The
microorganisms in the coral gastrovascular cavity may have roles in food digestion
and nutrient absorption like the gut microorganism of other organisms (Thompson
et al. 2014). A study by Agostini et al. (2012) measured the physical properties of a
gastric cavity of Galaxea fascicularis at different depths by using fiber-optic microsensors and observed elevated levels of vitamin B12, phosphate, and nitrogen species. They hypothesized that vitamin B12 may be produced by bacteria as a nutrient
for coral or Symbiodinium. Some coral-associated bacteria are photosynthetic in
nature that may provide the photosynthetic products in the absence of their photosynthetic partner such as during coral bleaching to help the coral animal to rejuvenate. For example, a study has shown the presence of Cyanobacteria in bleached
Oculina patagonica that supply photosynthesized products to the coral (Fine and
Loya 2002, 2004). Moreover, these bacteria are also the direct source of nutrition to
corals through bacterivory (Kushmaro and Kramarsky-Winter 2004).
16.4.1.2 Bacteria Required for Metamorphosis and Settlement
of Coral Larva
Metamorphosis and settlement are tightly coupled processes in corals wherein a
mobile planula larva transforms into a sedentary polyp animal. Emerging evidence shows that bacteria play an important role in this transformation process
of the coral animal. These studies have reported that the coral-associated bacteria
provide protection to the coral spawn or newly hatched larvae through their antimicrobial activity (Marquis et al. 2005). A study by Negri et al. (2001) showed
an increase in metamorphosis and larval settlement on crustose coralline algae
(CCA) surfaces that harbors a consortium of a complex community of bacteria.
Moreover, antibiotic treatment of larval cultures and rock surface showed sufficient inhibition for a larval settlement that signifies the importance of bacterial
activity (Huggett et al. 2006; Vieira et al. 2016). Various researches are enduring
to understand the mechanism behind the induction of metamorphosis and settlement of coral larva. Scientist suggests that this event may be induced by diffusible or potentially contact-mediated signals (Hadfield 2011; Dobretsov et al.
2013; Shikuma et al. 2014). For example, Tebben et al. (2011) isolated tetrabromopyrrole (TBP) from various Pseudoalteromonas strains isolated from CCA
surfaces, known to stimulate metamorphosis in Acropora millepora. In contrast,
a recent study reported that both natural and synthetic TBP induce metamorphosis as well as a coral larval settlement in Caribbean corals (Sneed et al. 2014).
16 Role of Bacteria in Coral Ecosystem
There are several pieces of evidence that support the role of bacteria in sulfur
cycle. For instance, in a study by Raina et al. (2009), they suggested the role of
coral-associated bacteria in providing sulfur to the host by degrading sulfur compounds such as dimethylsulfoniopropionate (DMSP), dimethyl sulfide (DMS), and
acrylic acid. Moreover, these sulfur compounds may have a role in structuring bacterial communities beneficial for the health of corals.
Coral-associated bacteria also participate in scavenging limiting nutrients such
as iron by siderophore production, amino acids, and vitamins for the host. The
microorganisms in the coral gastrovascular cavity may have roles in food digestion
and nutrient absorption like the gut microorganism of other organisms (Thompson
et al. 2014). A study by Agostini et al. (2012) measured the physical properties of a
gastric cavity of Galaxea fascicularis at different depths by using fiber-optic microsensors and observed elevated levels of vitamin B12, phosphate, and nitrogen species. They hypothesized that vitamin B12 may be produced by bacteria as a nutrient
for coral or Symbiodinium. Some coral-associated bacteria are photosynthetic in
nature that may provide the photosynthetic products in the absence of their photosynthetic partner such as during coral bleaching to help the coral animal to rejuvenate. For example, a study has shown the presence of Cyanobacteria in bleached
Oculina patagonica that supply photosynthesized products to the coral (Fine and
Loya 2002, 2004). Moreover, these bacteria are also the direct source of nutrition to
corals through bacterivory (Kushmaro and Kramarsky-Winter 2004).
16.4.1.2 Bacteria Required for Metamorphosis and Settlement
of Coral Larva
Metamorphosis and settlement are tightly coupled processes in corals wherein a
mobile planula larva transforms into a sedentary polyp animal. Emerging evidence shows that bacteria play an important role in this transformation process
of the coral animal. These studies have reported that the coral-associated bacteria
provide protection to the coral spawn or newly hatched larvae through their antimicrobial activity (Marquis et al. 2005). A study by Negri et al. (2001) showed
an increase in metamorphosis and larval settlement on crustose coralline algae
(CCA) surfaces that harbors a consortium of a complex community of bacteria.
Moreover, antibiotic treatment of larval cultures and rock surface showed sufficient inhibition for a larval settlement that signifies the importance of bacterial
activity (Huggett et al. 2006; Vieira et al. 2016). Various researches are enduring
to understand the mechanism behind the induction of metamorphosis and settlement of coral larva. Scientist suggests that this event may be induced by diffusible or potentially contact-mediated signals (Hadfield 2011; Dobretsov et al.
2013; Shikuma et al. 2014). For example, Tebben et al. (2011) isolated tetrabromopyrrole (TBP) from various Pseudoalteromonas strains isolated from CCA
surfaces, known to stimulate metamorphosis in Acropora millepora. In contrast,
a recent study reported that both natural and synthetic TBP induce metamorphosis as well as a coral larval settlement in Caribbean corals (Sneed et al. 2014).
16 Role of Bacteria in Coral Ecosystem